The visible universe just shrunk by 320 million light-years in all directions, updating a famous calculation that physicists first made 13 years ago.
If you’re trying to calculate the size of the cosmos, the speed of light — the fastest anything can go — is a tempting place to begin and end. You’d reason that since the Big Bang happened some 13.8 billion years ago, there’s a 13.8-billion-light-year radius marking the edge of what mere mortals could see.
As physicists have shown again and again over the past century, space is expanding faster and faster. There’s also a blinding glow of light that didn’t clear up until about 378,000 years after the Big Bang — an event called recombination (when particles finally cooled down enough to form the first atoms).
If you take expansion, recombination, and other variables into account, as physicist J. Richard Gott III and several of his colleagues did in 2003, you get an observable universe that’s roughly 45.66 billion light-years in radius — or 91.32 billion light-years wide (if diameter is your thing).
However, as physicists Paul Halpern and Nick Tomasello at the University of the Sciences explain in a post on Medium.com, that calculation was based on data from the WMAP satellite, which mapped the afterglow of the Big Bang— and that data is no longer the best around.
Swapping in newer, more refined data on the universe’s expansion from the European Space Agency’s Planck satellite, Halpern and Tomasello calculated that the observable edge of the universe is actually 0.7% smaller, or 45.34 billion light-years in radius.
Their paper with the new number-crunching will appear in an upcoming edition of the journal Advances in Astrophysics.
“A difference of 320 million light-years might be peanuts on the cosmic scale, but it does make our knowable universe a little bit cozier,” Tomasello wrote in the post.
A few things could stretch this observable limit a bit to 46.31 billion light-years, though — namely ghostly particles called neutrinos.
Neutrinos pass through ordinary matter as if it isn’t there, so they could have sailed right through the glow of recombination and may offer the earliest-ever view of the universe. But the same reason they could do that also makes them devilishly hard to detect, as any physicist with an underground neutrino-detecting laboratory can attest.